One of the most significant developments in semiconductor technology in recent years has been the increased use and importance of compound semiconductors, particularly the group III-V compounds composed of elements III and V of the periodic table such as gallium arsenide and indium phosphide. The band gap characteristics of III-V semiconductors make them particularly useful as photonic devices such as lasers, light emitting diodes (LEDs) and photodetectors. The high electron mobility of such materials also make them promising for the production of high speed electronic devices such as high speed integrated circuits. Photonic devices are often made by forming on the surface of the substrate a succession of epitaxial layers (an epitaxial layer is a layer of material deposited on a substrate such that it has a crystal structure that constitutes in effect an extension of the crystal structure of the substrate). For example, a laser based on indium gallium arsenide phosphide epitaxially grown on an indium phosphide substrate emits light in a wavelength range where absorption losses of silica based optical fibers is at a minimum. Also, modulation doped hetero structures grown on indium phosphide substrates such as In.sub.0.53 Ga.sub.0.47 As/InP have been used to make high speed integrated circuit devices.
Most useful III-V semiconductor devices require a low resistance or ohmic metal contact to the III-V material for applying electrical current to, or removing electrical current from, the device. Such ohmic contacts are usually made by evaporating a thin gold film onto the semiconductor substrate. While such gold films have reasonably good electrical characteristics, they frequently do not adhere well to the substrate and are often characterized by poor uniformity, reproducibility and reliability. Many of these problems probably result largely from the fact that the gold alloys to the III-V material and then diffuses into the substrate. The papers, "On the Formation of Binary Compounds in Au/InP System," A. Piotrowska et al., Journal of Applied Physics, Vol. 52, No. 8, August 1981, pp. 5112-5117, and "The Migration of Gold from the p-contact as a Source of Dark Spot Defects in InP/InGaAsP LED's," A. K. Chin et al., IEEE Transactions of Electronic Devices, Vol. ED-30, No. 4, April 1983, pp. 304-309, document the large diffusion depth of gold into the substrate and the problems that can result. The paper, "The Design and Realization of a High Reliability Semiconductor Laser for Single-Model Fiber-Optical Communication Links," A. R. Goodwin et al., Journal of Light Wave Technology, Vol. 6, No. 9, September 1988, pp. 1424-1434, further documents degradation due to the electromigration of gold into the device during operation. Furthermore, as described in "Interaction Between Zinc Metallization and Indium Phosphide," S. Nakahara et al., Solid-State Electronics, Vol. 27, No. 6, June 1984, pp. 557-564, dopant atoms added to the contact diffuse rapidly through indium phosphide, which can result in a reduction of the ohmic behavior. A high contact resistance not only results in low efficiency and reduces the speed of such devices, but also increases the temperature of the active region leading to a higher threshold current. Another consideration in designing ohmic contact is the desirability of using a metal that can be patterned by reactive ion etching (reactive ion etching or dry etching is a well-known technique for using ions of a plasma selectively to etch metal films).
Accordingly, there has been a long-felt need for a method for making ohmic contacts to III-V semiconductor substrates that are uniform, reproducible, reliable, adhere well, do not interfere with the electrical characteristics of the semiconductor substrate, and are amenable to reactive ion etching.